Up until now, a semiconductor laser pumped solid state laser apparatus has been known, such as the one shown in FIG. 6, in which a second harmonic generation crystal (=SHG element) 56 and a third harmonic generation crystal (=THG element) 55 are aligned in an optical resonator for the fundamental wave (See Patent Document 1.). The second harmonic generation crystal 56 outputs a second harmonic wave 2ω from a fundamental wave ω. The third harmonic generation crystal 55 outputs a third harmonic wave 3ω from the fundamental wave ω and the second harmonic wave 2ω.    [Patent Document 1] U.S. Pat. No. 7,016,389
In the above described conventional semiconductor laser pumped solid state laser apparatus, a type I nonlinear optical crystal is used as the SHG element 56 and a type II nonlinear optical crystal is used as the THG element 55. If a combination of nonlinear optical crystals other than the above combination is used in the optical resonator, for example, if a combination of type I crystal for SHG and a type I crystal for THG is used, it is necessary to mutually rotate the axes of the crystals by 45 degrees, which leads to a rotation of a polarization in the optical resonator due to the birefringence, resulting in an occurring of an unstable oscillation. For this reason, the nonlinear optical crystals that can be used are practically limited to the above combination.
In view of the above discussion, an object of the present invention is to provide a semiconductor laser pumped solid state laser apparatus in which it is possible to use a type I nonlinear optical crystal or a quasi phase matching element as the third harmonic generation crystal.